1. Field of the Invention
This invention is in the field of wax-type compositions suitable for use as pattern materials in the conventional mold making process for precision investment casting. The two major constituents of the composition are a mixture of palmitic and stearic acids, in combination with a compatible thermoplastic resin, with minor amounts of a wax, a fatty acid amide and at least one metal stearate.
2. Description Of The Prior Art
The precision investment casting process, known as the "lost wax" process has been used in the manufacture of molds for hundreds and perhaps thousands of years. Basically, the process consists in forming a pattern of a heat disposable material, usually formed of a wax or waxy material, forming a mold about the pattern by successively dipping the pattern into dispersions of finely divided ceramic particles, with intermediate application of relatively coarse, stuccoing particles. A shell-type mold is thus built up around the pattern and when the pattern is removed by means of firing in a furnace, steam autoclaving and the like, a shell mold results having a molding cavity which is an exact duplicate of the original pattern.
The British publication entitled "Pattern Materials and Their Use in Investment Casting" prepared by the B. I. C. T. A. Pattern Making Committee, has set forth a summary of the physical properties of pattern materials which are required in modern day usage of the lost wax process. For one, the wax must be safe to handle as a solid and must not give rise to noxious fumes upon melting or burning out. In addition, the wax should have a low ash content on the order of 0.05% or so. Good oxidation resistance is required since the molten pattern wax tends to oxidize slowly in the air. The waxy material must also be resistant to any organic solvent or alkali which may be present in the dipping slurries. Where the patterns are formed by injection molding, which is the usual technique, they must be capable of quick setting. In addition, the plasticity or ductility of the pattern material must be low at ambient temperatures so that patterns and assemblies do not sag under their own weight but at the same time, the pattern wax must not be brittle.
The pattern material must have sufficient strength to be handled during molding operations, and still possess a certain degree of resilience particularly in the areas of the sprues which must carry the weight of the patterns while the patterns are being dipped in the wet refractory slurries. Patterns must also be reasonably hard at room temperature so that the patterns can be handled throughout the assembly operations. They must also evidence an ability to produce strong welds so that an assembly of pattern clusters can be conveniently made.
The surface finish of a pattern must be good if faithful reproduction is to be achieved in the investment mold. Where a blend of materials is used as a pattern, materials must be compatible with each other, i.e., they must dissolve in each other to a point where separation will not occur when the mixture is held within a given temperature range. The thermal characteristics of the wax are also important, particularly the viscosity at the optimum injection temperature, and the expansion-contraction characteristics. Low shrinkage is particularly important if a precise mold cavity is to be obtained from the pattern.
As the precision investment mold making procedure has advanced, the patterns used in such mold making have increased in size and mass. Patterns for cores have become quite delicate to the point that they are difficult to inject. Going to multiple waxes has not been a satisfactory solution since this increases the quantity of injection equipment and lost machine time. As the part shape becomes more complicated, an increase in pattern repair and scrap results. The need still remains, therefore, for a pattern material which has a low injection temperature, a low coefficient of expansion, and insubstantial cavitation after injection into a pattern die.
There are examples in the prior art illustrating the use of various fillers to wax compositions in order to reduce the contraction of the wax. While this technique has met with some success, it has introduced some problems of its own. Carbon black, for example, is not particularly satisfactory as a filler because of the difficulty in handling finely divided particles. Polystyrene spheres have been used but they tend to coagulate in the molten wax at high melt temperatures. The use of urea has met with some success, but this material absorbs water and breaks down at elevated temperatures. Carbohydrates such as sucrose are limited in their applicability because they are affected by moisture. Organic acids such as adipic acid are sometimes satisfactory, but the acidity of these acids affects ceramic molds, and they tend to gum up in the molten wax at high temperatures.
In the patent art, Feagin U.S. Pat. No. 3,316,105 describes a pattern wax composition in which the shrinkage characteristics are improved, i.e., lowered by the addition to the wax of certain diamides of polyamino compounds.
Larson U.S. Pat. No. 3,801,335 describes a pattern wax composition in which there is a filler material consisting of particles of pentaerythritol or an oligomer thereof.
Speyer U.S. Pat. No. 3,854,962, assigned to the assignee of the present application, describes a pattern composition containing one or more waxes and a combustible polyhydric alcohol in combination with an organic fatty acid which suspends the polyhydric alcohol filler.